The term “battery” originally meant a plurality of electrochemical cells connected in series in a housing. However, single electrochemical cells are nowadays frequently also referred to as batteries. During the discharge of a battery, an energy-supplying chemical reaction made up to two electrically coupled, but physically separate subreactions takes place. One subreaction taking place at a comparatively low redox potential proceeds at the negative electrode, while a subreaction at a comparatively higher redox potential proceeds at the positive electrode. During discharge, electrons are liberated at the negative electrode by an oxidation process, resulting in flow of electrons via an external load to the positive electrode which takes up a corresponding quantity of electrons. A reduction process thus takes place at the positive electrode. At the same time, an ion current corresponding to the electrode reaction occurs within the cell. This ion current is achieved by an ion-conductive electrolyte. In secondary cells and batteries, this discharge reaction is reversible, and it is thus possible to reverse the transformation of chemical energy into electric energy which occurs during discharge. If the terms anode and cathode are used in this context, the electrodes are generally named according to their discharge function. In such cells, the negative electrode is thus the anode, and the positive electrode is the cathode.
The extractable charge of a cell, which depends on the discharge conditions to a great extent, is referred to as capacity (unit Ah). The specific charge (unit Ah/Kg) or charge density (unit Ah/L) is a measure for the number of electrons and/or ions liberated or received per unit mass or volume, and thus for the storage capacity of electrodes and batteries. Accordingly, in this context reference is made to the specific capacity of electrodes and batteries which is also indicated in the unit Ah/kg. A great potential difference between negative and positive electrode combined with electrode materials of high specific charge or charge density results in high values for the specific energy (unit Wh/kg) or the energy density (unit Wh/L). When operating a battery, the rates of electron transfer and ion transfer inside the battery, in particular the rate of ion transfer at the phase interfaces within the electrodes, limit the power. The relevant properties of batteries in this regard can be taken from the key figures specific power (unit W/kg) and power density (unit WA).
Among secondary cells and batteries, lithium ion batteries achieve comparatively high energy densities. These batteries generally have composite electrodes which comprise electrochemically active components together with electrochemically inactive components. Possible electrochemically active components (often also referred to as active materials) for lithium ion batteries are in principle all materials which can take up lithium ions and release them again. In this regard, concerning the negative electrode, in particular particles based on carbon such as graphitic carbon or non-graphitic carbon materials capable of intercalating lithium, are state of the art. Furthermore, also metallic or semi-metallic materials which can be alloyed with lithium can be used. For instance, the elements tin, antimony and silicon are capable of forming intermetallic phases with lithium. Concerning the positive electrode, industrially applied active materials at present comprise lithium cobalt oxide (LiCoO2), LiMn2O4 spinel (LiMn2O), lithium iron phosphate (LiFePO4) as well as derivatives such as, for example, LiNi1/3Mn1/3Co1/3O2 or LiMnPO4. All electrochemically active materials are generally contained in the electrodes in the form of particles.
As electrochemically inactive components, mention may be made first and foremost of electrode binders and current collectors. Electrons are supplied to or discharged from the electrodes via current collectors. On the one hand, electrode binders ensure the mechanical stability of the electrodes and, on the other hand, the contacting of the particles made of electrochemically active material among themselves as well as to the current collector. Conductivity-improving additives, which can likewise be subsumed under the collective term “electrochemically inactive components” can contribute to improved electrical contact of the electrochemically active particles with the current collectors. All electrochemically inactive components should at least be electrochemically stable in the potential range of the respective electrode and should be further chemically inert towards common electrolyte solutions. Common electrolyte solutions are solutions of lithium salts such as lithium hexafluorophosphate, in organic solvents such as ethers and esters of carbonic acid.
The aforementioned carbon based active materials for the negative electrode allow reversible, specific capacities of up to ca. 372 Ah/kg. An even significantly greater storage capacity of up to 4200 Ah/kg is exhibited by the aforementioned metallic or semi-metallic materials that can be alloyed with lithium. In contrast, the capacities of the aforementioned cathode materials are only 110 Ah/kg to 250 Ah/kg. Therefore, attempts are made to balance the materials for the positive electrode and the negative electrode in terms of quantity to match the actual capacities of the electrodes in a most optimal way.
In this context, it is of particular importance that already during the first charge/discharge cycle of secondary lithium ion cells (the so-called “formation”), a cover layer is generated on the surface of the electrochemically active materials in the anode (see D. Aurbach, H. Teller, M. Koltypin, E. Levi, Journal of Power Sources 2003, 119-121, 2). The cover layer is referred to as “Solid Electrolyte Interphase” (SEI) and generally mainly consists of electrolyte decomposition products as well as of a certain amount of lithium which correspondingly is no longer available for further charge/discharge reactions. Ideally, the SEI is only permeable for extremely small lithium ions, and prevents further direct contact of the electrolyte solution with the electrochemically active material in the anode (see B. V. Ratnakumar, M. C. Smart, S. Surampudi, Journal of Power Sources 2001, 97-98, 137). To an extent, generation of the SEI has positive effects. However, the loss of mobile lithium due to SEI-generation has a negative impact. Usually, during the first charging process, there is a loss of approximately 10% to 35% of the mobile lithium depending on the type and the quality of the applied active material and electrolyte solution. The achievable capacity likewise decreases by that percentage. Those losses due to formation have to be considered when balancing the anode and the cathode.
During subsequent cycles, generally there are only slight lithium losses, respectively. However, upon a higher number of cycles, the small lithium losses sum up to an important, maybe even the most important parameter for cell aging (see J. Vetter, P. Novak, M. R. Wagner, C. Veit, K.-C. Möller, J. O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, Journal of Power Sources, 2005, 269-281, 147). The thickness of the SEI layer on the surface of the electrochemically active materials is ever more increasing, frequently resulting in a significant increase of cell impedance. In addition, the increasing losses of mobile lithium have to be considered. The effects lead to a continuously progressing decrease in capacity and power of the affected cell. The cell thus ages.
The use of over-lithiated cathode materials in combination with anodes based on carbon materials can cause a reduction of that phenomena, however, to the detriment of safety, as the case may be. Whether there is a safety problem, depends on the ratio of maximum capacity of the anode to the capacity of the anode after the formation. For example, the maximum specific capacity of anodes on graphite basis is at 372 mAh/g. In the case of a 15% loss of mobile lithium due to SEI generation in the course of formation, it is thus theoretically possible to combine the anode with a cathode containing an amount of mobile lithium ions which corresponds to the equivalent of 372+15%˜427 mAh/g. However, if the lithium loss during formation is less pronounced, an undesired deposition of metallic lithium at the anode can occur. The anodes of lithium ion batteries are generally over-dimensioned to prevent this.
Furthermore, it is problematic that lithiation of the aforementioned active materials is accompanied by a significant increase in volume. Thus, the volume of graphite particles can increase by up to 10% when taking up lithium ions. The volume increase is even greater in the case of the aforementioned metallic and semi-metallic storage materials. For example, when lithiating tin, antimony and silicon, the volumetric expansion during the first charging cycle can be up to 300%. Upon releasing the lithium ions, the volume of the respective active materials decreases again, which causes high mechanical stresses within the particles made of active material and, as the case may be, to a shifting within the electrode structure. In some cases, the associated mechanical stress of the electrodes to a significant extent leads to contact losses between adjacent particles made of active material, which has an adverse effect on the capacity and the life cycle of the affected battery.
It could therefore be helpful to provide lithium ion batteries with improved aging performance, where the above problems do not occur or occur to a lesser extent.